Although communication systems usually known as local area networks (`LANs`) have reached considerable sophistication, they are known to have various short-comings which prevent their use as the number of users increase. The development of asynchronous data transfer promises to be a versatile and convenient solution to the difficulties of providing a large scale communication network which enables concurrent use by a great multiplicity of users. At the present time there is a large number of local area networks in use and, notwithstanding their limitations, they are convenient for small organizations or local data transfer. Furthermore, it is expected that local area networks conforming to the agreed formats and protocols will remain in widespread use for many years to come. In order to use the vast base of existing software that is particular to local area networks yet obtain the advantages of data transfer in asynchronous transfer mode, it is desirable to provide a service or mode of operation which may be called local area network emulation, in which, among other aspects, end systems such as work-stations, servers, bridges etc can be connected to an asynchronous transfer mode network while the software which the local area network uses functions as if it were used in an ordinary local area network. In other words, the asynchronous transfer mode system is transparent to the users of the local area network or networks to which it is connected. Each emulated local area network (ELAN) is composed of a set of `local area network emulation clients`, each of which is an entity which performs data forwarding, address resolution and other control functions and which may be part of an asynchronous transfer mode end station, and a single local area network emulation service in accordance with the ATM Forum: Lan Emulation over ATM Specification.
One function that an asynchronous transfer mode link needs to provide in emulating local area networks is a virtual circuit, normally called a Broadcast and Unknown Server (BUS) circuit or channel, which enables packets to be broadcast to all the members of an emulated lan. It is also a means of transmitting unicast packets before the resolution of an addresses of an incoming packet which has a destination address expressed in local area network terms (i.e. an ethernet address or a token ring address) and which needs eventually to be expressed in terms of a virtual circuit number. The resolution of the destination address (often termed `media access control address) requires, in general, the broadcast (using a control channel) of a request for address resolution to all the members of the respective emulated lan and an appropriate reply in a manner which is not directly relevant to the present invention. It will be apparent that this necessary technique is liable to cause the `BUS` to become a potential restriction on the flow of signal traffic, because it can become too heavily loaded.